As well known, a glass bulb in a cathode ray tube (CRT) used in a TV set or a computer monitor basically includes a panel through which picture images are shown, a conical funnel bonded to the back of the panel and a tubular neck bonded to an apex portion of the conical funnel. The panel is constituted by a face portion for displaying images, a skirt portion extending backward from a perimeter of the face portion and having a seal edge on its back end, and a blend radius portion integrally joining the skirt portion to the face portion. The funnel is divided into a body portion having a seal edge and a yoke portion extending backward from the body portion. The seal edge of the body portion is bonded to the seal edge of the skirt portion, and the neck is bonded to the yoke portion.
Such panel, funnel and neck are made of glass, wherein particularly the panel and the funnel are manufactured by pressing molten glass called a glass gob into predetermined dimensions and shapes. The pressed panel is cooled down by forced air draft, so that the panel receives its final form. Afterwards the panel is admitted to a pin sealing machine. On the pin sealing machine the studs (also called pin) are melted into the panel. Then, stresses present in the panel are relaxed by heat treatment in an annealing lehr and the panel goes through inspection procedure to be a product.
In the normal annealing process, the panel is cooled down to a temperature at 520° C., i.e., the annealing point, or below before being entered to the annealing lehr. The annealing point is the temperature at which most of stresses present in the panel are relaxed if the panel is kept in the annealing lehr at this temperature for about 15 minutes. The panel cooled down to the annealing point or below is conveyed through the annealing lehr whose temperature is maintained at about 520° C., and then cooled down to room temperature. It takes about 30 minutes to complete the annealing process. The stress present in the panel is classified into compressive stress and tensile stress. After the annealing process, the residual compressive stress at a surface of the panel is in the range of 0 to −3 MPa and the residual tensile stresses at inner surfaces of corner portions are equal to or less than +10 MPa (a minus sign (−) in front of a stress value indicates the compressive stress and a plus sign (+), the tensile stress). However, the normal annealing process is not suitable for a glass bulb maker, which mass-produces panels, as it lessens the productivity and increases the production cost.
Recently, conventional spherical panels have been rapidly replaced by flat panels because of customers' increasing demand for high definition and large-size screen. When compared to the spherical panels, the flat panels offer numerous advantages. For example, they can reduce image distortion, minimize eye fatigue and provide a wide range of visibility. By the way, as a cathode ray tube becomes flattened and enlarged for the flat and large-size screen, thickness and weight of a glass bulb are increased to secure its mechanical strength. The increase in weight of the glass bulb is due to the increase in weight of a flat panel, and the increase in weight of the flat panel degrades its formability and bondability resulting in a fall of glass bulb productivity.
Therefore, glass bulb makers and cathode ray tube makers have been actively carrying out researches on weight reduction of the glass bulb for improving productivity by shortening annealing time and for reducing thickness and weight of glass bulb as well as on cathode ray tube for the flat and large-size screen.
As a method for compensating for structural weakness of the glass bulb caused by its weight reduction, physical strengthening method is used to form a compressive stress layer on a surface of a panel in a thickness of about 20% of the thickness of the panel. In the physical strengthening method, the panel is thermally treated in the annealing lehr whose highest temperature is less than the annealing point, i.e., 520° C. Then, the panel is cooled down to room temperature, so that residual stresses whose levels are higher than that of the panel thermally treated by the normal annealing process are imparted thereto.
However, the physical strengthening method causes a permanent tensile stress in the panel as the panel is cooled down non-uniformly for non-uniform thickness distribution of the panel of a complicated three dimensional structure. Further, tensile stress makes glass vulnerable to a mechanical impact, so defects are easily formed in the panel having tensile stress even by a little mechanical impact. Accordingly, there is a drawback that the panel thermally treated by the physical strengthening method is readily broken due to thermal stress in a frit sealing furnace used in manufacture of a cathode ray tube. In addition, as the compressive stress value of the panel becomes higher, the tensile stresses at inner surfaces of corner portions in a diagonal direction of the panel are increased.
There has been reported no panel which is capable of satisfying requirements for weight reduction of the glass bulb and breakage prevention of the glass bulb due to residual tensile stress or membrane stress, i.e., the stress present in inner surface of corner portion of the panel thermally treated in the annealing lehr.